Shortness of breath 8 weeks post mechanical AVR surgery

I had my surgery 8 weeks ago (On-X valve) and then needed a pacemaker 5 weeks ago. I am going to rehab and can do treadmill, bike, etc. at increasing speed and resistance with no SOB. But if I walk up the stairs, or bend over and then stand up and walk a few steps, I get terribly short of breath (gasping). Has anyone else experienced this? Is it normal at this stage? Thanks.